Embodiments of inventive concepts disclosed herein relate to the field of heading or pitch determination for vehicles. More particularly, embodiments of inventive concepts disclosed herein relate to systems and methods of providing enhanced integrity for global navigation satellite system (GNSS) derived attitude parameters including but not limited to heading angle and pitch angle.
A heading angle estimate is used in vehicular applications to predict hazards, provide direction, and control the vehicle. For example, an aircraft heading angle estimate in passenger-carrying, civil certified aircraft is generally provided by an inertial reference system (IRS), a gyrocompassing attitude and heading reference system (gyrocompassing AHRS), a directional gyro (DG) AHRS, an AHRS with a magnetically referenced heading, or combinations thereof. The IRS and gyrocompassing AHRS provide very accurate heading angle estimates (the specification can be 0.4 degrees, 95%). When redundant IRS or gyrocompassing AHRS are used, these systems provide high accuracy/high integrity heading angle estimate, sufficient for such applications as head-up display (HUD) approach guidance. However, IRS and gyrocompassing AHRS are expensive.
Magnetically referenced heading measurement is much less accurate (the specification can be 2 degrees, 95% in wings level non-accelerating flight, where magnetic declination is below 70 degrees). This accuracy is not sufficient for HUD approach guidance. Magnetometers are also subject to magnetic field disturbances, which could cause large heading errors. Such disturbances could be caused by aircraft equipment, or by ferrous elements in the runway, for example.
DG function of an AHRS is typically used with a magnetically referenced AHRS, when magnetic field measurement is known to be or detected as inaccurate. In this case heading accuracy from DG AHRS can be no better than magnetic heading accuracy and it degrades with time based on DG drift rate. Safety objectives for heading indications on Part 25 aircraft are provided in FAA AC 25-11B. See Section 4.7.4.4 of AC 25-11B. Accordingly, there is a need for a low cost heading estimator that has sufficient accuracy and integrity for HUD approach guidance and other aircraft operations.